1. Field of the Invention
The present invention relates to a method of making a mandrel for use with blind rivets.
2. Prior Art
FIGS. 7(a) and 7(b) show a typical example of the prior art blind rivets. Blind rivet 1 comprises a cylindrical rivet body 2 and a mandrel 5 inserted therein, wherein the rivet body has at one end thereof a flange-shaped rivet head 3. A bore 4 opened through the rivet body receives the mandrel 5, which comprises a mandrel body 5a having at its end a mandrel head 6, as well as a constricted portion 7 adjacent thereto. The mandrel head 6 and constricted portion 7 are located opposite to the rivet 3 located at the one end the rivet body 2. The mandrel head 6 in use is plastically deformed and exponded at the other open end of the rivet body. The mandrel body will subsequently be broken off the mandrel head, at its constricted portion 7. The reference numerals 8 and 9 denote of objects which are fastened together.
Mandrels 5 for the blind rivets 1 are made of a metal rod, for example of an inexpensive iron rod made by using in general a nail manufacturing apparatus. The round mandrel heads 6 are formed in a one-shot manner by a punching press, directly without performing any preliminary pressing step. It is however noted that the iron rods severed in the nail manufacturing apparatus generally have their extremities wedge-shaped or pyramid-shaped. Usually, those iron rods having such extremities are directly pressed in a spherical cavity of the punching press, in a state such that their ends substantially protrude from a split die. Those ends tend often to slant noticeably or swell irregularly in a radial direction during the pressing process, so that the pressed heads 6 are rendered somewhat offset with respect to the axes of the mandrel bodies 5a. Thus, a peripheral portion of mandrel head is likely to be uneven in thickness and/or shape around the axis of each mandrel body. If, in use, such an irregularly formed head 6 of the mandrel 5 presses or expands the open end 2a of the rivet body 2, then the latter will be deformed non-uniformly as shown in FIG. 7(b). Consequently, the mandrel head 6 will tend to slip off the rivet body, or the rivet body 2 will collapse during the riveting operation, and/or the rivet as a whole will fail therefore because the required strength of connection, due to such an irregular deformation cannot be achieved.